Currently, navigation devices that provide travelling guidance to a destination are mounted in many vehicles. Moreover, in addition to manual driving for travelling on the basis of driving operation by a driver, development of drive assistance technologies of a vehicle are under way in recent years such as travelling automatically along a preset route (or a route to a destination guided by a navigation device) without requiring driving operation by a driver.
Demonstration experiments of automatic driving on public roads by vehicle manufacturers and the like have already begun. These experiments have unveiled various problems that may occur when automatic driving is performed, and a large number of proposals have been made to solve these problems.
For example, a proposal has been made on a travel assistance device that performs, when the automatic driving mode is started, switching from a manual driving mode to an automatic driving mode after a transition mode in which cancellation of the automatic driving mode is easy as compared with the automatic driving mode, detects override control by a driver when the automatic driving mode is cancelled, performs switching from the automatic driving mode to the manual driving mode when it is determined that the safety of a vehicle at the time of switching to the manual driving mode can be secured, and cancellation of the transition mode is performed in a case where override operation by the driver is detected (see Patent Document 1 for example).
Also, there has been proposed a drive assistance device that notifies the driver that switching from automatic driving to manual driving is recommended in a case where more than a certain number of surrounding vehicles travel, the ratio of the manually driven vehicles becomes high during automatic driving of the own vehicle, and automatic driving becomes difficult (see Patent Document 2 for example).